Just fire the thruster:
2.2 Launch The default Atlantis does not implement an autopilot or launch computer, so you need to adjust the ascent to orbit manually. (You can download 3rd party addons which provide more realistic launch and flight operation modes). Here is a quick checklist you can follow to get Atlantis into orbit: Fire main engines at 100%. SRBs are ignited automatically when main engines reach 95%. SRBs are not controlled manually. Once ignited, they cannot be shut off. During launch, attitude is controlled via SRB thrust vectoring. Roll shuttle for required heading, and decrease pitch during ascent for required orbit insertion. SRBs separate automatically at T+2:06min. In an emergency, SRBs can be jettisoned manually with . Ascent continues with Orbiter main engines. Throttle down as required for 3g max acceleration. Tank separates at T+8:58min (alt 110km) when empty, or manually with . After tank separation, orbiter switches to OMS (orbital maneuvering system) using internal tanks, for final orbit insertion. Attitude thrusters (RCS – reaction control system) are activated.